This invention relates to the synchronization of Orthogonal Frequency Division Multiplex (OFDM) signals, such as may be used for broadcasting digital television signals in the uhf (ultra high frequency) bands, or for digital audio broadcasting (DAB).
The form of OFDM signal proposed for this purpose consists of data signals and reference information modulated as QPSK (quadrature phase shift keying) or QAM (quadrature amplitude modulation) on to several thousand individual carriers, evenly spaced in frequency and occupying a total bandwidth of several Megahertz in the uhf spectrum. The data signal on each carrier has a relatively long symbol period and this, in part, gives the signal its good performance in conditions of multipath propagation. The multipath performance is further enhanced by the inclusion of a guard interval in which a portion of the modulated signal waveform taken from the end of each symbol is also included at the beginning of the same symbol period. Different fractions of the basic symbol period, such as 1/32, 1/16,1/8, or 1/4, can be used in this way to provide immunity to multipath distortion of increasingly long delays.
More specifically, each symbol is extended by a period T.sub.G (the guard interval) which precedes the "useful" or "active" symbol period T.sub.S , so that the whole symbol now lasts T.sub.T in total. T.sub.S is the reciprocal of the carrier spacing f.sub.S , and is the duration of the time domain signal produced or analysed by the FFT (fast Fourier transform) in the transmitter and receiver respectively.
Each carrier is continuous over the boundary between the guard interval and the active part of the same symbol, keeping the same amplitude and phase. If the signal at complex baseband is considered, with all the carrier frequencies not only spaced f.sub.S, but also equal to multiples of f.sub.S, then the signal in the guard interval is effectively a copy of the segment of the signal occupying the last T.sub.G 's worth of the active part, as shown in FIG. 8 of the accompanying drawings. It follows that the signal has the same value at any two instants which are separated by T.sub.S but lie within the same symbol.
Specific proposals for synchronization of OFDM receiving apparatus have been published for example in European Patent Applications EP-A-0 653 858 and 0 608 024, and International Patent Applications WO95/07581, WO95/05042 and WO95/03656.
The principal requirement for synchronization in a receiver is to obtain from the signal waveform a reliable time synchronization pulse related to the start of the symbol period. Such a pulse could then be used to start, at the correct position in the waveform, the process of Fourier transformation which accomplishes a major portion of the demodulation process. A second requirement for synchronization is to lock the digital sampling clock in the receiver to an appropriately chosen harmonic of the symbol period. However, the modulated OFDM waveform produced by adding together all the modulated carriers is essentially noise-like in nature and contains no obvious features such as regular pulses which could be used to synchronize the circuitry of a receiver.
Because of this, we have previously proposed techniques for synchronization which are based on correlation of the signal with a version of itself delayed by the basic symbol period. The similarity between the portion included to form the guard interval and the final part of the basic symbol is then shown as a region of net correlation, while the ramainder of the symbol period shows no correlation. Even so, the correlated waveform still reflects the noise-like nature of the signal waveform and can be impaired by signal distortions, so it is necessary to process the signal further to obtain reliable synchronization.
Our European patent application No. 96307964.5, publication No. 0 772 332 (publication date May 7, 1997), describes the use of a correlator with a filter which exploits the periodicity of the waveform to form a complex symbol pulse and then uses the argument of the pulse to obtain frequency control for a local oscillator. In addition, the modulus of the pulse signal is used to derive a pulse related to the start of the symbol period and to derive a signal to control the clock frequency in the demodulator. A complex integrate-and-dump technique is included in the clock loop to suppress interference.